Increasing the emissions of greenhouse gases is among the factors affected the speed of occurrence of climate change during recent decades. In present study, first using time series data of 2006-2012 andRCM-PRECIS simulation model, the impacts of greenhouse gases emission on climatic variables of temperature and precipitation was investigated under different scenarios in watersheds of QazvinProvince. Then, the ordinary least squares (OLS) method and regression analysis were used to assess theimpacts of climatic variables of temperature and precipitation on the selected products yield. Afterwards,considering the results of regression analysis in positive mathematical programing (PMP) model, theamount of the created variation in supply and demand of irrigation water and agricultural output inwatersheds of Qazvin Province was investigated. The results showed that emission of greenhouse gasesunder scenarios A, B, and C affects the climatic variables of temperature and precipitation about 0.43 to1.27 °C and -14.1 to 1.31 mm respectively. This case changes the selected products yield in the surface of each river basin of Qazvin Province. Change in yield affects acreage of agricultural crops by about -10.51to 3.17 percent, the amount of irrigation water supply by about -10.4 to 1.64 percent, and the amount ofirrigation water demand by about 1.60 to 7.35 percent. Moreover, the results showed that maximum andminimum decrease in the gap between supply and demand of irrigation water happens in Kharroud andShahroud watersheds by about 9.20 and 1.82 percent respectively. With estimating the gap betweendemand and supply of irrigation water, one can adopt the appropriate decisions for sustainable waterresources in watersheds of Qazvin Province
